With the widespread growth of optical fiber telecommunications, networks are being used to distribute information between an increasing number of terminals (nodes). The increasing demand however presents a dilemma. On one hand, the number of divisions a given signal can go through while propagating through a network is limited by the minimal signal quality or power necessary at the terminal level. On the other hand, the number of terminals served by a network should be maximal to reach as many users as possible.
Presently, network capacity is generally increased through the provision of amplifiers throughout the network. However, amplifiers are active components and have many drawbacks, in that they demand electrical power supplies, they increase the probability of a breakdown in the network and necessitate regular maintenance, thereby requiring the frequent on-site involvement of qualified personnel. The use of active components therefore considerably increases the operation cost of the network.
In view of the above, the industry has been leaning towards the concept of Passive Optical Networks (PON), where active components are solely located at a central office and the general distribution network includes only passive elements.
An erbium-doped fiber amplifier (EDFA) includes active components, such as the pump, the electrical feed and the control electronics, and passive components, such as the doped fiber, the dividers, isolators and wavelength-division multiplexers (WDM). It is possible to separate the active and passive components while still performing the amplifying function. For example, the active elements may be placed in a central office and the pump sent along a network segment to a location where amplification is needed and where the passive elements are located. This technique, described in U.S. Pat. No. 5,321,707 (Huber), is referred to as “remote pumping”.
When taking into account the energy transfer balance between the pump and the signal, a centralized amplification using a power amplifier or booster in the central office may give better results than distributed pumping. Centralized amplification however has its limitations. Firstly, the maximal power of signals injected in the fiber must be limited to avoid detrimental non-linear effects. Secondly, one must consider the often-unpredictable evolution of the network's characteristics, such as asymmetry between various branches, and increases in the size and number of users. In particular, this last factor necessitates a solution that allows a modular evolution of the network, independent of its size or architecture. With the often asymmetrical evolution of networks, where given branches may expand to include more ramifications than others, it is capital to have a solution that does compensate for losses independently of the state of the network.